Industrial paper mills, carpet warehouses, newspaper print shops, textile mills, and other industrial environments use rolls or spools that dispense or take-up a sheet of material (or web) during processing. There are numerous techniques to track the material remaining on the roll and replace the rolls when necessary. Different mechanical or electronic sensor devices may track how much material is either wound onto or unwound from the roll, indicating the rate of change and determining the time before the roll is empty or full and must be replaced.
For example, some devices sense a reduced roll diameter as it dispenses the sheet material and cooperate with a web splicing apparatus in which a new roll is inserted onto the mandrel and spliced into the old sheet once the sensing mechanism determines that a new roll is required. Roll diameter sensing mechanisms are also used in torque regulated tension control systems where the measured diameter of the roll provides an inertia compensation for acceleration and deceleration and a torque or tension set point. For example, in a speed regulated tension control system, the measured diameter is used for inertia calculations that adjust a speed loop gain and velocity set point for a drive motor.
A mechanical lay-on roller is a more simple tension control system where a lay-on roller on a pivoting follower arm contacts the outside diameter of the roll. A sensor is mounted to a pivot point on the follower arm. The measured angle of the follower arm from the sensor is converted into the roll diameter. The sensor tracks how much material is on the roll and indicates the rate of change and determines the time before the roll is empty. Although these lay-on roller systems are simple, the roller must contact the surface of the material on the roll, which can be an issue in industrial environments because of slippage and inaccurate diameter readings. Also, mechanical systems require extensive maintenance almost every day in harsh industrial environments.
Ultrasonic sensors have been used to measure the roll diameter. The system determines the rate of change of material on the roll and the time before the roll becomes full or empty or full and must be replaced. Ultrasonic sensors are not advantageous in some industrial environments, however, because the sheet of material on the roll may absorb the sound waves and give inaccurate measurements.
Other systems use a laser to emit a 6 millimeter (mm) wide beam, visible red light that bounces off the roll and scatters some of its light through a sensor's receiving lens into a photodiode. The time interval between the transmitted and received electrical pulses is used to calculate the distance to the roll using the speed of light and time-of-flight calculations. These visible red light laser systems use special diffused mode sensors because the laser is a visible red light having a wide beam of about 6 millimeters to ensure that a wide surface on the roll is illuminated. Although these systems may operate well in a clean industrial environment using the time-of-flight calculations, the wide diameter beam with the red visible light may need special diffuse mode sensors that may not be adequate in harsh industrial environments.